Conduction and transport numbers in metacomposites MeWO4 • WO3 (Me = Ca, Sr, Ba)

The conduction and transport numbers of charge carriers for composites in the systems MeWO4_ WO3 (Me = Ca, Sr, Ba) are studied as a function of the temperature and the activity of oxygen in a gas phase. The transport numbers are determined by an emf method and are estimated from dependences of conductance on the activity of oxygen in a gas phase. The deficiencies of the given procedure as applied to investigation of properties of composite phases are analyzed. The materials under study are classified with a class of metacomposites. The concentration intervals of the ionic, mixed, and electronic conductions are determined. The conduction of composites of compositions (100 - x)MeWO4 (.) xWO(3) is predominantly ionic at x <= 10 (Me = Ca), x <= 60 (Me = Sr), and x <= 45 (Me = Ba). The obtained data are explained in the framework of a model that represents a composite as a distributed matrix system where a film of surface phase MeW-s with a high mobility of oxygen ions plays the role of a connected matrix. It is presumed that the surface phase MeW-s possesses double-sided surface activity (alpha(MeW-s) <= alpha(WO3) , alpha(MeWO4)) and a higher mobility with respect to MeWO4 and WO3. The discovered anomalies of dependences (sigma(-)(O2) (T) and partial dependences sigma(O)2-, sigma(el)(T) are explained by a change in the stoichiometry, morphology, and the degree of connectedness of surface phase MeW-s caused by with a change in the temperature and composition of composites.